The subject matter disclosed herein relates generally to a system for increasing the current capacity of a distributed motor drive system and, more specifically, to a capacitance module and an extension module mountable proximate to an inverter in the distributed motor drive system.
As is known to those skilled in the art, a motor drive is used to convert energy from a first form to a second form. The input voltage may be a DC voltage having a fixed amplitude or a single phase or multi-phase AC voltage having a fixed amplitude and frequency. The output voltage may be an AC voltage having a variable frequency and/or amplitude, where the motor drive controls the frequency and amplitude supplied to the motor such that the motor rotates at a desired speed or produces a desired torque. According to one common configuration, the motor drive includes a front end, configured to convert an AC input voltage to a DC voltage present on a DC bus, and an inverter, configured to convert the DC voltage from the DC bus to the AC voltage provided to the motor.
Some controlled systems, such as a process line or a machining center, may include multiple motors, each controlling a different axis of motion. The controlled system receives AC power from the utility grid, which is, in turn distributed to each of the motors. It may be advantageous to provide a single AC-to-DC converter, or rectifier, having a power rating sufficient to provide power for all of the motors in the controlled system, to convert the AC power to DC power for distribution via a shared DC bus. Although the single rectifier module may require more space due to its power rating than a rectifier sized for one of the motors, the overall space required may be reduced by not requiring a separate rectifier for each motor. Each motor may then have an associated motor drive which includes an inverter connected to the shared DC bus. The motor drive controls operation of the inverter to provide AC power to the motor to achieve a desired operation of the motor. Each motor drive may be mounted proximate to the rectifier module or optionally, may be distributed about the controlled system proximate to the motor being controlled by the motor drive.
However, shared DC bus systems have not been fully met without incurring various disadvantages. Because the DC bus is shared between inverters, the conductors, or at least a portion of the conductors, for the DC bus must be sized to handle the total current capacity of all inverters connected to the DC bus. If an inverter is located adjacent to the rectifier module, a DC bus bar may be used to connect the two modules. The bus bar is a preformed, rigid conductor having a fixed routing path, often between the two modules and is often rigidly connected to each module. Such connection may be suitable for adjacent modules. If, however, an inverter is located remotely from the rectifier module, it typically requires a DC bus cable. A DC bus cable may be routed in a flexible manner as required between modules. Increased current capacity in a DC bus cable is obtained, by utilizing a larger wire gauge. However, as the wire gauge for the DC bus cable increases, the weight of the cable similarly increases and flexibility for routing is reduced. The increased weight and reduced flexibility of the cable generates increased strain on the connector for the motor drive to which the DC bus cable is connected. The physical limitations on connecting the DC bus cable to the motor drive may restrict the current rating for a shared DC bus and, therefore, restrict the number of motor drives that may be connected to the shared DC bus.
Thus, it would be desirable to provide an improved connector between a DC bus cable and motor drive to provide increased current capacity on the shared DC bus.
As is known in the art, motor drives typically include a capacitance connected in parallel across the DC bus. The capacitance helps reduce the magnitude of ripple on the DC bus resulting from converting the AC voltage to a DC voltage and helps maintain a constant amplitude of voltage on the DC bus. The capacitance additionally acts to reduce ripple voltage on the DC bus resulting from the controlled switching within the inverter to convert the DC voltage to the desired AC voltage waveform. In a motor drive that includes both the rectifier and the inverter, each motor drive also includes a DC bus capacitance typically within the motor drive. However, in the shared DC bus system, the majority of the DC bus capacitance may be located in the rectifier module. The individual motor drives typically have a nominal capacitance and rely primarily on the capacitance in the rectifier module to reduce the ripple voltage on the DC bus resulting from the switching in each motor drive.
As previously indicated, when a motor drive is located remotely from the rectifier module, a DC bus cable typically connects the shared DC bus between the rectifier module and the motor drive. However, inductance in the DC bus cable isolates the motor drive from the capacitance in the rectifier module. As the distance of the DC bus cable increases, the inductance in the DC bus cable similarly increases. The increased inductance creates greater isolation of the motor drives from the capacitance in the rectifier module. As a result, an increase in ripple current and/or harmonic content at the switching frequency or multiples of the switching frequency for the motor drive is present on the shared DC bus. These undesirable current components limit the amplitude of desirable DC bus current that may be conducted on the DC bus cable without exceeding the current rating for the DC bus cable and will reduce the life of the capacitors connected to the DC bus, potentially resulting in premature failure of a motor drive.
Thus, it would be desirable to provide a system that reduced the ripple and/or harmonic current present on the DC bus as a result of inverter switching.